The mouse epidermal keratinocyte cell line MKDC4, previously established in this project, was further studied for growth in two specially formulated media: (a) LEP/MK2, containing 0.1 mg/ml bovine serum albumin (BSA) and 0.5% bovine pituitary extract (BPE, equivalent to 0.09 mg protein/ml), and (b) LEP/MK4, containing no BPE and 1.5 mg/ml BSA. Growth in the LEP/MK4 medium was found to support colony formation but failed to support extended growth or subculture. The MKDC4 cell line in LEP/MK2 medium was further studied in the presence of inorganic particu- lates. Ferric oxide not only did not decrease colony forming efficiency, but at certain doses induced a stimulation of cell growth and colony formation. Crystalline silica particles of various types, in contrast, induced a marked dosedependent inhibition of these parameters. The MKDC4-derived cell line, 2057C, selected for its ability to grow continuously in the presence of high levels of transforming growth factor beta (1 ng/ml), was reestablished for further mechanism studies. Methods were explored for the establishment of cultures of pulmonary alveolar type II epithelial cells from F344 rats, the target cell for silica-- induced carcinogenesis in vivo.